Our 5 series was in a fender bender a few years ago with Geico, had a similar issue, wanted to use non oem parts. Talked to our agent and he said there is an extra charge for being able to use only oem specific parts. I had never heard of this and told him I was never offered or explained the clause of this in the policy. It only added about $50/yr. I said of course I would have been paying that if it had been offered so added it to my policy and agreed to allow the oem parts. Their original quote was less than $3k, but ended up around $7k. Hopefully they will be understanding and make an exception for you. Good Luck